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| United States Patent |
5,675,246
|
|
Jones
|
October 7, 1997
|
Current flow indicator
Abstract
An apparatus connectable between a current source and an electrical load
for indicating current flow through the load above a threshold level. The
indicator includes two negative temperature coefficient (NTC) thermistors,
and a neon indicator lamp. The first thermistor is connected directly in
series with the electrical load and the second thermistor is connected in
series with the neon lamp. The thermistors are packaged together by an
epoxy resin, which thermally couples the thermistors so that both
thermistors operate at substantially identical temperatures. Current flow
above the load's operational threshold level causes the first thermistor
to heat to an equilibrium temperature well above the ambient temperature.
At this elevated equilibrium temperature, both resistors have extremely
low resistance values. The low resistance value of the second thermistor
permits maximum current flow to the neon lamp, which illuminates the lamp.
As the current flow through the load drops, the self-heating experienced
by the first thermistor decreases and the resistance value of the second
thermistor increases. When the current flow through the load falls below
the load's operational threshold level, the resistance value of the second
thermistor is sufficiently high to limit the current flow to the neon lamp
and prevent illumination. The failure of the neon lamp to illuminate
provides a visual indication that the current flow to load is below the
load's operational threshold.
| Inventors:
|
Jones; Thaddeus M. (Bremen, IN)
|
| Assignee:
|
MSX, Inc. (South Bend, IN)
|
| Appl. No.:
|
619259 |
| Filed:
|
March 18, 1996 |
| Current U.S. Class: |
324/122; 340/595 |
| Intern'l Class: |
G01R 019/00; G08B 017/00 |
| Field of Search: |
324/122,133
340/595,661,662,664
361/106
|
References Cited
U.S. Patent Documents
| 3840834 | Oct., 1974 | Obenhaus et al. | 361/106.
|
| 3872355 | Mar., 1975 | Klein et al. | 361/106.
|
| 4238812 | Dec., 1980 | Middleman et al. | 361/106.
|
| 4310837 | Jan., 1982 | Kornrumpf et al. | 340/598.
|
Primary Examiner: Karlsen; Ernest F.
Attorney, Agent or Firm: Crump; R. Tracy
Claims
I claim:
1. In an electrical system including a current source and an electrical
load, an apparatus connectable between said current source and said load
for indicating current flow through said load above a predetermined
threshold level, said apparatus comprising:
a first and second thermistor, each thermistor electrically connected
between said current source and said load, light means connected in series
to said second thermistor and responsive to current flow through said
second thermistor for illuminating when the voltage applied thereto is
above a predetermined value, and bonding means intimately connecting said
first and second thermistor for thermally coupling said first and second
thermistor so that said first and second thermistors have substantially
identical temperatures,
said first thermistor connected in series between said current source and
said load, said second thermistor having a resistance value which varies
inversely with temperature and being connected in series to said light
means, said first thermistor dissipates thermal energy to heat said second
thermistor so that the resistance value of said second thermistor permits
sufficient voltage to said light means to illuminate said light means only
when said current flow through said load is above said threshold level.
2. The apparatus of claim 1 wherein said bonding means is an epoxy resin.
3. The apparatus of claim 1 wherein said light means is a neon lamp.
Description
This invention relates to an apparatus used to indicate current flow
through an electrical load, and in particular, an apparatus, which is
connected as a separate component between the current source and the load
for indicating current flow through a load above a threshold level.
BACKGROUND OF THE INVENTION
In many electrical applications, current flow indication provides a means
for determining the operational state of electrical loads and control
devices. This method is commonly used in commercial and industrial
temperature and heater control systems. In electrical temperature and
heater control systems, such as those used for freeze protection, the
absence of current flow to electrical heaters at sub-freezing temperatures
is a serious problem, which requires immediate attention. Loss of the
current flow in electrical freeze protection systems can result in damage
to plumbing and piping, as well as, the failure of wet fire protection
sprinklers.
Conventional current flow indicating devices employ current transformers
and electrical components. Because of the expense of current transformers
and the other electrical components needed for conventional current flow
indication, current indication is not practical for small electrical
systems. Electrical systems designed for consumer markets often forego
current indication features due to the expense of such a feature.
Consequently, a simple, reliable, and cost effective current flow
indicator is desirable. In particular, a current flow indicator which can
be connected as a separate component between the current source and the
load in a simple electrical application, would be desirable for a variety
of consumer applications, such as electrical freeze protection systems.
SUMMARY OF THE INVENTION
The apparatus of this invention can be connected as a separate component in
series between the current source and the electrical load of any
electrical system to effectively and inexpensively indicate current flow
through the load above a threshold level. The current flow indicator of
this invention is particularly well suited for use in electrical heater
control applications and heating cable applications. The indicator can be
readily connected as a separate component between the current source
(power line outlet) and the heating cable.
The current flow indicator of this invention includes two negative
temperature coefficient (NTC) thermistors, a neon indicator lamp and two
resistors, which form a voltage divider. The first thermistor is connected
directly in series with the electrical load and the second thermistor is
connected in series with the neon lamp. The first thermistor is of the
type which are commonly used for inrush current limiting in electronic
power supplies. Current flow causes significant self-heating in this type
of thermistor. The second thermistor is of the type used for temperature
sensing. This type of thermistor is selected because the current flow
during any operational condition does not cause sufficient self heating to
effect its resistance value. The thermistors are packaged together by a
suitable epoxy resin, which thermally couples the thermistors. The epoxy
resin conducts heat so that both thermistors have substantially identical
temperatures. The neon lamp operates from current flow through the second
thermistor and illuminates only when the current flow through the load is
above a threshold operational level. The second thermistor and other
resistors form a voltage divider, which attenuates the voltage supply to
the neon lamp during operation.
Current flow above the operational threshold required by the electrical
load causes the first thermistor to heat to an equilibrium temperature
well above the ambient temperature (100.degree. C., or more, above the
ambient temperature). Since the thermistors are thermally coupled by epoxy
resin, the thermal energy from the first thermistor is transferred to the
second thermistor. At this elevated equilibrium temperature, both
resistors have extremely low resistance values. The low resistance value
of the second thermistor permits maximum current flow to the neon lamp.
Consequently, when the current flows through the load above the load's
operational threshold level, the resistance value of the second thermistor
is at a minimum and the neon lamp reaches its full intensity. As the
current flow through the load drops, the self-heating experienced by the
first thermistor decreases and the resistance value of the second
thermistor increases. When the current flow through the load falls below
the load's operational threshold level, the resistance value of the second
thermistor is sufficient to limit the current flow to the neon lamp to
prevent illumination. The failure of the neon lamp to illuminate provides
a visual indication that the current flow to load is below the load's
operational threshold.
Accordingly, an advantage of this invention is to provide for an apparatus
which effectively and inexpensively indicates current flow through a load
above a threshold level.
Another advantage of this invention is that the current flow indicator of
this invention can be connected as a separate component between the
current source and the load to provide a visual indication of load
operation.
Another advantage of the invention is that the current flow indicator can
be used to indicate heater operation in temperature and heater control
applications, such as freeze protection systems.
Other advantages will become apparent upon a reading of the following
description.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention has been depicted for illustrative
purposes only wherein:
FIG. 1 is a schematic of the current flow indicator of this invention used
in a typical electrical heater application; and
FIG. 2 is a perspective view of the current flow indicator of this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment herein described is not intended to be exhaustive
or to limit the invention to the precise form disclosed. It is chosen and
described to best explain the invention so that others skilled in the art
might utilize its teachings.
FIGS. 1 shows the current flow indicator 10 of this invention used in a
typical electrical application, i.e., an electrical heating system. The
current flow indicator of this invention is used to indicate the presence
of current flow above a threshold value in any electrical system
application. As shown, indicator 10 is connected as a separate component
between a current source 2 and a load 4 by wire leads 3, 5. In this
description, current source 2 is a power line outlet and load 4 is a
conventional electrical heating device and its controller, although the
teachings of this invention are applicable to any current source and
electrical load. Although indicator 10 is shown in the figures and
described herein as a separate component connected between the current
source and the load, the circuitry of the indicator can be readily
incorporated into the circuitry at the current source of the load.
As shown in FIG. 1, the circuitry of indicator 10 includes: two negative
temperature coefficient (NTC) thermistors 12 and 14; a neon lamp 20; and
two resistors 16 and 18. As shown in FIG. 2, the electrical circuitry of
indicator 10 is enclosed by a protective outer housing 30. The outer
housing 30 is constructed of any suitable material and can assume any
suitable configuration. Preferably, housing 11 is constructed of a
transparent material or has a window 31 (FIG. 2.) through which neon lamp
20 is visible. Indicator 10 also includes wire leads 32 and connector
plugs 34 for convenient connection between current source 2 and load 4.
As is commonly known in the electrical arts, the resistance of NTC
thermistors varies inversely with temperature. Preferably, thermistor 12
is of the type which is commonly used for inrush current limiting in
electronic power supplies. This type of thermistor is selected because
high current flow causes significant self heating in the order of one to
five watts. Thermistor 12 is also selected to have an extremely low
resistance value at high temperatures, generally in the range of
100.degree. C., or more, above the ambient temperature. Thermistor 14 is
of the type used for temperature sensing. This type of thermistor is
selected because the current flow during any operational condition does
not cause sufficient self heating to effect its resistance value.
Thermistor 14 also is selected to have a significantly higher resistance
value at high temperatures than thermistor 12.
As shown in FIG. 1, thermistor 12 is connected directly in series with load
4. Thermistor 14, resistor 16 and neon lamp 20 are connected in parallel
to thermistor 12 and load 4. Thermistors 12 and 14 are intimately packaged
together by an epoxy resin (designated by numeral 13). Collectively,
thermistors 12 and 14 in the epoxy resin are referred to herein as
thermistor package 15. The epoxy resin has a superior thermal conductivity
to that of air, and thermally couples thermistors 12 and 14 together.
Epoxy resin 13 facilitates the conduction of thermal energy between
thermistors 12 and 14 so that both thermistors operate at substantially
identical temperatures. Any suitable bonding material with the desired
thermal conductivity properties, such as high temperature wax, can be
substituted for the epoxy resin.
Indicator 10 uses neon lamp as the means for visually indicating the
presence of current flow, but any suitable light source, such as a light
emitting diode (LED), can be incorporated within the teaching of this
invention. Neon lamp 20 operates from current flow through thermistor 14
and glows only when the current flow through the load is above a threshold
operational level. Generally, the ionization potential of neon lamps is
such that they will not glow unless the instantaneous voltage applied to
it is above a threshold voltage of approximately 65 volts. Resistor 18 is
connected in parallel to neon lamp 20 to act as a shunt. Thermistor 14 and
resistors 16 and 18 form a voltage divider, which attenuates the voltage
supply to neon lamp 20.
Increased current flow through load 4 increases the self-heating of
thermistor 12. Since thermistors 12 and 14 are thermally coupled by epoxy
resin 13, the thermal energy generated by current flow through thermistor
12 is transferred to thermistor 14. Generally, a high current flow is
required to operate load 4. Current flow above the operational threshold
required by load 4 causes thermistor 12 to heat to an equilibrium
temperature well above the ambient temperature (100.degree. C. above the
ambient temperature). At this elevated equilibrium temperature, both
resistors 12 and 14 have extremely low resistance values. The low
resistance value of thermistor 12 permits maximum current flow to load 4
and has a negligible effect on the operation of load 4. The low resistance
value of thermistor 14 also permits maximum current flow to neon lamp 20.
When the full operational current flows through thermistor 12, the
combined resistance of thermistor 14 and resistor 16 is at a minimum and
neon indicator 20 reaches its full intensity. Resistor 16 limits the
current supplied to neon lamp 20 to an appropriate value when current flow
through load 4 is above the load's operational threshold level.
Decreased current flow through load 4 decreases the self-heating
experienced by thermistor 12, which decreases the temperature of
thermistor package 15. As the temperature of thermistor package 15 falls,
the resistance value of thermistor 14 increases. Once the current flow
through load 4 falls below the load's operational threshold level, the
resistance value of thermistor 14 is sufficient to limit the current flow
to neon lamp 20 and prevent illumination. At such time, the voltage
divider formed by thermistor 14 and resistors 16 and 18 attenuates the
supply voltage to a value below the ionization potential of the neon
indicator (generally below 65 volts peak) and lamp 20 does not illuminate.
Failure of neon lamp 20 to illuminate provides a visual indication that
the current flow to load 4 is below the load's operational threshold.
It is understood that the above description does not limit the invention to
the details given, but may be modified within the scope of the following
claims.
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